1. Field of the Invention
The present invention relates to a multiplex coherent Raman spectroscopic detector and method for generating and detecting coherent Raman radiation from a sample. More specifically, the present invention relates to a multiplex coherent Raman radiation detector and method for generating and detecting coherent Raman radiation scattered by the components of an unknown sample and using the coherent Raman radiation to determine the identity of the sample""s constituents.
The present invention also relates to an apparatus and method for illuminating a gaseous sample with broadband light with a continuous range of more than 3000 wavenumbers and with a narrowband light having a bandwidth of less than 1 wavenumber, and preferably about 0.003 wavenumbers to produce the entire gas phase vibrational Raman spectrum of the sample, thereby permitting accurate identification of the sample.
In addition, the present invention relates to an apparatus and method for increasing the intensity of the backward-propagating, phase-conjugate, coherent Raman radiation produced by a Raman cell. Moreover, the present invention also relates to an apparatus and method for using this enhanced, backward-propagating, phase-conjugate, coherent Raman radiation to drive a device capable of producing broadband light of more than 3000 wavenumbers.
2. Description of Related Art
In many fields, such as scientific research, industrial research, and forensics, it is often necessary to identify the chemical composition of an unknown sample. This task is often performed by first isolating the different compounds in the sample, and then applying an identification technique to each isolated compound. One standard method for isolating unknown compounds is called gas chromatography, where the unknown sample is transformed into a gas, if not already in the gaseous state, and the various compounds in the gas are separated due to their differing gaseous properties, such as polarity. Once the compounds are isolated, they may be identified. The simplest way to identify the compounds is by noting the time it takes for each compound to pass through the gas chromatograph, since different compounds take different amounts of time to do so. But this method is limited to samples where much is known about the components. A more powerful method for identifying isolated compounds examines the intensity of different wavelengths of light emitted, transmitted, reflected, or scattered by the compound. This technique, called spectroscopy, works if each compound emits, transmits, reflects, or scatters light differently and if the spectroscopic instrument has sufficient spectral resolution to detect these differences. More specifically, different chemical compounds emit, transmit, reflect, or scatter different wavelengths of light with differing intensities. A graph or picture of such data is called the spectrum of that compound. Different types of spectroscopy reproduce the spectrum of a compound over different wavelengths and/or under different conditions. If the type of spectroscopy used provides a unique spectrum for each chemical compound, an unknown compound can be identified by producing its spectrum (for example, by illuminating the compound and measuring the light reflected, scattered, or emitted therefrom) and comparing its spectrum with the spectra of known compounds. As a result, gas chromatographs, which isolate compounds from a sample, are often used with spectrometers, which identify the compounds once they are isolated.
One popular type of spectroscopy detector used with gas chromatographs requires the gas isolated by the gas chromatograph to be embedded in or condensed onto a substrate before spectroscopic examination. Such detectors provide advantages, such as low detection limits, but are complicated because they require the isolated gas to be condensed, trapped, or adsorbed onto a substrate. In addition, such detectors suffer from unwanted effects such as nearest-neighbor effects, sample decomposition, and a slow detection speed. As a result, detectors that operate xe2x80x9con the flyxe2x80x9d with little or no sample modification are often faster and freer from unwanted effects.
One type of frequently-used xe2x80x9con the flyxe2x80x9d spectroscopy is infrared spectroscopy. But infrared spectroscopy is sometimes unable to accurately determine the identity of an unknown sample because certain characteristics of some samples (i.e., those with spectra that are highly state-(phase) dependent and those that produce strong rotational side bands in the infrared light absorbed by the sample that cause a loss of spectral resolution) reduce its accuracy. Furthermore, certain molecules, such as homonuclear diatomics, have no infrared spectrum, and optical components designed to direct and process the infrared light used in an infrared spectrometer are often inferior to the optical components designed for use in the visible spectrum.
A type of spectroscopy that is less susceptible to these problems is called Raman spectroscopy. In this type of spectroscopy, light in the visible wavelength region or the near-visible wavelength region is projected onto a sample and a small fraction of this light is scattered in all directions by the sample and is measured. The light is scattered because the molecules of the sample inelastically scatter the light due to the vibrational or rotational motions in the molecules of the sample. Such scattered light is of two types: light whose wavelength is not shifted, which is called Rayleigh scattering, and light whose wavelength is shifted, which is called Raman scattering. The Raman scattered light is much less intense than the Rayleigh scattered light. Since the Raman scattered light is scattered and shifted in wavelength because of the vibration of molecules of the sample, a graph of the Raman scattered light from a sample is called the vibrational Raman spectrum of the sample and provides information about the internal vibrational motion of the molecules of the sample. Moreover, the entire vibrational Raman spectrum of each compound (which is approximately 3000 wavenumbers wide) is unique to that compound. As a result, unknown compounds can be identified by their vibrational Raman spectrum. But, the intensity of the Raman spectrum must be sufficiently strong to be detected by currently-developed detectors with a high signal-to-noise ratio, and the entire Raman vibrational spectrum, covering a range of at least 3000 wavenumbers (indicating a large number of wavelengths of light are measured) must be produced. If only a partial Raman vibrational spectrum is produced, the identity of the compound may not be determined with high accuracy, since many compounds can share the same partial Raman vibrational spectrum. When Raman spectroscopy is used to detect gases, such as those isolated by a gas chromatograph, it is called gas phase Raman spectroscopy.
Gas phase Raman spectroscopy provides several advantages over gas phase infrared spectroscopy. First, Raman spectroscopy is less susceptible to phase transitions in the sample and to unwanted broadening of scattered or absorbed light due to rotational sidebands, so species identification may be more accurate using Raman spectroscopy. Second, Raman spectroscopy can be used to identify more types of molecules than infrared spectroscopy, since certain molecules do not appear in infrared spectroscopy, while all molecules will appear in Raman spectroscopy. Third, several advanced techniques are available with Raman spectroscopy that improve its accuracy and generate additional, valuable data not available in infrared spectroscopy, including resonance Raman spectroscopy, surface enhanced Raman spectroscopy, and coherent Raman spectroscopy. Finally, the optical components commercially for use in the visible region are often superior to those available for use in the infrared region. For example, extremely sensitive and rapid multichannel detectors are available in the visible region but not in the infrared region.
But gas phase Raman spectroscopy suffers its own problems. The density of molecules in the gaseous sample is so low that long collection times (minutes or hours) are needed in order to generate Raman spectra. This problem precludes the use of conventional Raman spectroscopy as an on-the-fly detector for gas chromatography, since in gas chromatography, different gases emerge from the gas chromatograph every few minutes, seconds, or less.
In order to overcome this problem, Roth and Kiefer, in xe2x80x9cSurface-Enhanced Raman Spectroscopy as a Detection Method in Gas Chromatography,xe2x80x9d Applied Spectroscopy vol. 48, 1994, 1193-1195, explored the potential use of surface enhanced Raman spectroscopy (SERS). Surface enhancement can be used to increase the strength of the Raman signal, thereby reducing the time required to obtain spectra. Later, Carron and Kennedy published the first paper showing actual chromatograms using a SERS detector in xe2x80x9cMolecular-Specific Chromatographic Detector Using Modified SERS Substrates,xe2x80x9d Analytical Chemistry vol. 67, 1995, 3353-3356. Their method requires that the sample be trapped onto a substrate that attracts specific molecules based on their function groups and enhances them. This method offers high sensitivity and specificity. But it also suffers important disadvantages including the domination of the spectra by the substrate instead of the sample, the lack of universality of the technique (not all molecules will strongly adsorb onto a given substrate, and not all molecules will be enhanced), the frequent replacement of the substrate, and the possible decomposition of the sample or possible change of the sample upon adsorption onto the substrate.
To solve these problems with gas phase Raman spectroscopy, coherent Raman spectroscopy was developed in the early 1960s. Unlike conventional Raman spectroscopy and SERS, coherent Raman spectroscopy uses two or more pulsed lasers having sufficiently high peak intensities to cause a certain nonlinear optical effect in the sample that generates an intense, coherent beam in one direction. In contrast, in conventional Raman spectroscopy and in surface enhanced Raman spectroscopy, the signal is weakly scattered in all directions. This technique is described in xe2x80x9cMultiplex Coherent Anti-Stokes Raman Spectroscopy by use of a Nearly Degenerate Broadband Optical Parametric Oscillatorxe2x80x9d, Applied Optics, vol. 38, no. 27, pp. 5894-5898, Sep. 20, 1999 by Peter C. Chen, Candace C. Joyner, and Michael Burns-Kaurin, and xe2x80x9cImproved Scanning Range for Coherent Anti-Stokes Raman Spectroscopy Using A Tunable Optical Parametric Oscillatorxe2x80x9d, Analytical Chemistry, col. 68, no. 17, pp. 3068-3071, Sep. 1, 1996 by Peter Chen, both of which are incorporated by reference herein.
Coherent Raman spectroscopy is of two typesxe2x80x94scanned coherent Raman spectroscopy and multiplex coherent Raman spectroscopy. Scanned coherent Raman spectroscopy uses narrowband tunable lasers. This method gradually changes the frequency of one or more laser beams aimed at a sample, while using equipment to monitor the size of a coherent Raman beam produced by the sample when irradiated by the frequency-changing laser beams. But with this approach, the length of time required to generate a single spectrum is long. In contrast, multiplex coherent Raman spectroscopy, which uses a combination of narrowband and broadband lasers, allows data to be generated very quickly (in as little as one or a few laser pulses). In the past, a primary limitation of the multiplex technique has been that the bandwidth of the lasers has not been suitable to cover the entire vibrational spectrum with high spectral resolution. As a result, it has not been possible to achieve highly accurate identification of all samples, since the entire vibrational Raman spectrum could not be produced with high spectral resolution.
Thus, there is a need for a multiplex coherent Raman spectrometer and multiplex coherent Raman spectroscopy method that can rapidly produce a vibrational Raman spectrum of a sample so that it covers the entire vibrational region with high spectral resolution, thereby improving the accuracy of sample identification. More specifically, there is a need for a multiplex coherent Raman spectrometer and spectroscopy method that can rapidly produce the entire vibrational Raman spectrum of approximately 3000 wavenumbers with sub-wavenumber resolution, thereby permitting highly accurate sample identification.
It is an object of the present invention to provide a multiplex coherent Raman spectroscopy detector and method that can produce a vibrational Raman spectrum of a sample covering more than 1000 wavenumbers, thereby increasing the accuracy of sample identification.
It is a further object of the present invention to illuminate a sample with broadband illumination of sufficient bandwidth that the sample will scatter coherent Raman light to produce a gas phase vibrational Raman spectrum of a sample covering more than 1000 wavenumbers, and preferably at least 3000 wavenumbers, thereby increasing the accuracy of sample identification.
It is still another object of the present invention to increase the intensity of backward-propagating, phase-conjugate, coherent Raman radiation produced by a Raman cell, and preferably to increase the intensity to provide a sufficiently strong input beam for pumping or driving an optical parametric oscillator to produce a substantially stable output.
According to one aspect, the present invention that achieves at least one of these objectives relates to a multiplex coherent Raman spectrometer and spectroscopy method for rapidly detecting and identifying individual components of a chemical mixture separated by a separations technique, such as gas chromatography. The spectrometer and method increase the accuracy with which a variety of compounds are identified because they comprise means, elements, and steps to produce a gas phase vibrational Raman spectrum of an unknown sample gas of more than 1000 wavenumbers, and preferably can do so rapidly (within one or a few laser pulses), with a high signal-to-noise ratio and without any gaps in the spectrum. Preferably, the spectrometer and method accurately identify a variety of compounds because they comprise means, elements, and steps to produce the entire gas phase vibrational Raman spectrum of an unknown sample gas covering at least 3000 wavenumbers.
According to another aspect, the present invention provides an element, a step, or means that drive a broadband-beam-producing device to simultaneously illuminate the sample with a stable broadband beam of more than 1000 wavenumbers bandwidth, and preferably more than 3000 wavenumbers bandwidth and with a narrowband beam of less than 1 wavenumber bandwidth and preferably about 0.003 wavenumbers bandwidth of sufficient intensity to produce a gas phase vibrational Raman spectrum of the sample of more than 1000 wavenumbers, and preferably more than 3000 with a spectral resolution of less than 1 wavenumber and preferably about 0.003 wavenumbers rapidly (within one or a few laser pulses) with a high signal-to-noise ratio and without any gaps in the spectrum.
The element, means, and step for driving such a broadband-beam producing device can comprise a hydrogen-filled Raman cell, tilted by less than 2.2 degrees with respect to an input beam entering the sample-filled Raman cell, to produce a high-intensity, backward-stimulated, coherent Raman beam of 683 nm. More generally, this element, means, or step will produce such a high-intensity, backward-stimulated, coherent Raman beam of 683 nm when the hydrogen-filled Raman cell is tilted with respect to an input beam up to (but not exceeding) the angle at which less than the entire input beam enters a hole in the hydrogen-filled Raman cell, so that the focal point of the entire input beam in the hydrogen-filled Raman cell collides with a side wall on the inside of the hydrogen-filled Raman cell, as shown in FIG. 5. This 683 nm beam can be used to drive a broadband-beam producing device, such as a degenerate optical parametric oscillator to produce a stable broadband beam of 1100-1700 nm that covers a continuous range of 3200 wavenumbers. This broadband beam is then combined with a narrowband beam of 532 nm having a bandwidth of less than 1 wavenumber and preferably about 0.003 wavenumbers and focused into a heated windowless sample-filled Raman cell that receives gases (i.e. the sample) separated by a gas chromatograph. When these gases are illuminated with the combined broadband and narrowband beams, they emit coherent Raman radiation. This Raman radiation is of sufficient intensity and bandwidth that when it is filtered and then sent to a monochromator with multichannel detection, complete vibrational Raman spectra of at least 3000 wavenumbers is produced from one or a few laser pulses, without any gaps in the vibrational Raman spectra and with a high signal-to-noise ratio.
Other objects and features of the present invention will become more apparent upon consideration of the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings.